In thermal ink-jet printing, droplets of ink are selectably ejected from a plurality of drop ejectors in a printhead. The ejectors are operated in accordance with digital instructions to create a desired image on a print sheet moving past the printhead. The printhead may move back and forth relative to the sheet in a typewriter fashion, or it may be of a size extending across the entire width of a sheet, to place the image on a sheet in a single pass.
The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated by a heating element disposed on a surface within the channel. This rapid vaporization of the ink adjacent the channel creates a bubble which causes a quantity of liquid ink to be ejected through an opening associated with the channel to the print sheet. The process of rapid vaporization creating a bubble is generally known as "nucleation."
One common design of an ink-jet printhead is known as a "sideshooter." In a sideshooter design, the channels forming the ejectors are formed between two silicon chips, generally known as a heater chip and a channel plate. The heater chip includes a main surface having defined therein a number of selectably actuable heating elements, usually one heating element per ejector. The channel plate is bonded to the heater chip, and has defined therein a set of grooves, one groove for each ejector. Together, the heater chip and channel plate form a set of nozzles, with one heating element in the heater chip corresponding to each channel in the channel plate, resulting in a set of tubes in which a heating element is exposed within each tube.
In known commercial designs of such a sideshooter printhead, the channel plate is formed from crystalline silicon, and the channels are formed by orientation-dependent etching (ODE) to form V-shaped grooves in a main surface of the silicon. These V-shaped grooves correspond to natural crystal planes in the original silicon wafer, and are readily made, because the channels are naturally self-limiting in the etching process. When the channel plate with the V-shaped groove is bonded to a heater chip, the resulting channels or nozzles are triangular in cross-section, with the surface of the heater chip forming the third side of the triangle in addition to the straight sides formed by the V-groove of the channel plate. While this architecture provides many advantages in manufacture, the use of triangular-cross-section ejectors limits the cross-sectional area of the ejectors and can lead to practical problems such as unpredictable directionality of ejected droplets. It is therefore desirable to provide channels which are generally closer to a round shape in cross-section.